Lung Compliance Notes

Lung Compliance

Lung compliance refers to the degree of lung distensibility or elasticity. A compliant lung exhibits a high degree of distensibility and elasticity, whereas decreased lung compliance indicates that the lungs are not as elastic or distensible.

Types of Lung Compliance

There are two types of lung compliance:

• Static Lung Compliance

• Dynamic Lung Compliance

Static lung compliance is generally more relevant and important in clinical settings.

Dynamic Lung Compliance

The formula for dynamic lung compliance is:

$Compliance = {{Change } in Volume \over {Change } in Pressure} = {\Delta V \over \Delta P}$

Where:

• Volume is measured in milliliters (mL).

• Pressure is measured in centimeters of water pressure (cmH2O).

Understanding the Formula

The change in volume $(\Delta V)$ and the change in pressure $(\Delta P)$ each involve two variables:

${\Delta V = V1 - V2}$
${\Delta P = P1 - P2}$

• V1: Tidal volume during inspiration (typically 500 mL).

• V2: Tidal volume remaining at the end of expiration (ideally zero, as the full tidal volume should be exhaled).

• P1: Peak inspiratory pressure (the highest pressure during inspiration).

• P2: Pressure remaining in the lungs during end-exhalation, which is the Positive End-Expiratory Pressure (PEEP).

Example Calculation

Let's assume:

• V1 = 500 mL

• V2 = 0 mL

• P1 = 22 cmH2O (peak inspiratory pressure)

• P2 = 5 cmH2O (PEEP)

Using the formula, we get:

$Compliance = {500 - 0 \over 22 - 5} = {500 \over 17} \approx 29$

The result is 29 mL/cmH2O

Interpretation

This means that for every 1 cmH2O of pressure, 29 mL of air can be delivered into the patient's lungs.

Impact of Stiff Lungs

If the patient has stiff lungs (e.g., due to COVID-19), the equation changes. For example:

• V1 = 500 mL

• V2 = 0 mL

• P1 = 36 cmH2O (peak inspiratory pressure)

• P2 = 8 cmH2O (PEEP)

$Compliance = {500 - 0 \over 36 - 8} = {500 \over 28} \approx 17. 9$

In this case, the compliance is approximately 17.9 mL/cmH2O, indicating reduced lung compliance.

Differences Between Dynamic and Static Lung Compliance

Dynamic Lung Compliance: Takes into account the full inspiratory and expiratory cycle, including air movement. However, it is not as accurate for measuring lung elasticity because it factors in air resistance, such as airway resistance due to a small endotracheal tube. According to Poussway's Law, a smaller diameter tube increases airway resistance.

Static Lung Compliance: Calculated during no air movement, which eliminates airway resistance from the equation. The formula is the same, but P1 is the plateau pressure instead of the peak inspiratory pressure.

Static Lung Compliance Calculation

$Compliance = {\Delta V \over \Delta P} = {V1 - V2 \over P1 - P2}$

Where:

• V1: Tidal volume during inspiration (e.g., 500 mL).

• V2: Tidal volume at the end of expiration (0 mL).

• P1: Plateau pressure (measured during an inspiratory hold).

• P2: PEEP.

Example

Let's say:

• V1 = 500 mL

• V2 = 0 mL

• P1 = 15 cmH2O (plateau pressure)

• P2 = 5 cmH2O (PEEP)

$Compliance = {500 - 0 \over 15 - 5} = {500 \over 10} = 50$

Compliance is 50 mL/cmH2O

Plateau Pressure

Plateau pressure is measured during an inspiratory hold, where the ventilator holds the breath for approximately three seconds, allowing gas to distribute across the lung fields. Ideally, plateau pressure should be below 30 cmH2O. If the lungs are highly elastic, the peak pressure will drop significantly when the inspiratory hold is performed.

Normal Lung Compliance

For a healthy human, static lung compliance should be above 50 mL/cmH2O, ideally between 70 and 80 mL/cmH2O. A value less than 50 mL/cmH2O indicates reduced lung compliance due to conditions such as cystic fibrosis, COVID-19, or severe pneumonia. Loss of surfactant can also cause atelectasis, further reducing lung compliance.

In summary, static lung compliance is preferred for accuracy because it measures lung elasticity during no air movement, using plateau pressure (P1) in the calculation. Dynamic lung compliance, while still providing a measure of compliance, uses peak inspiratory pressure (P1) and is influenced by air movement and airway resistance.